feat(chat): add session pinning feature with database migration

- Add `pinned` column to chat_sessions table with default FALSE
- Implement database migration for existing installations using PRAGMA table_info
- Create composite index for pinned + updated_at sorting
- Add pin/unpin API endpoints (PUT/DELETE /sessions/{id}/pin)
- Update session listing to sort by pinned status then updated_at
- Extend ChatSessionsRepository with pin_session method
- Update API documentation to reflect new pinning functionality

The feature allows users to pin important chat sessions to the top of the list. Sessions are sorted with pinned sessions first (by updated_at), followed by regular sessions (by updated_at). Database migration ensures backward compatibility with existing installations.

gns3server/api/routes/controller/chat.py

@@ -323,3 +323,79 @@ async def rename_session(
         )
 
     return schemas.ChatSession(**session)
+
+
+@router.put(
+    "/sessions/{session_id}/pin",
+    response_model=schemas.ChatSession,
+    summary="Pin a chat session",
+    description="Pin a chat session to the top of the list."
+)
+async def pin_session(
+    session_id: str,
+    project: Project = Depends(dep_project),
+    current_user: schemas.User = Depends(get_current_active_user),
+) -> schemas.ChatSession:
+    """
+    Pin a chat session.
+    """
+
+    # Check if project is opened
+    if project.status != "opened":
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail=f"Project must be opened to pin chat sessions. Current status: {project.status}"
+        )
+
+    # Get AgentService for this project
+    agent_manager = await get_project_agent_manager()
+    agent_service = await agent_manager.get_agent(str(project.id), project.path)
+
+    # Pin session
+    session = await agent_service.pin_session(session_id, pinned=True)
+
+    if not session:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"Session '{session_id}' not found"
+        )
+
+    return schemas.ChatSession(**session)
+
+
+@router.delete(
+    "/sessions/{session_id}/pin",
+    response_model=schemas.ChatSession,
+    summary="Unpin a chat session",
+    description="Unpin a chat session from the top of the list."
+)
+async def unpin_session(
+    session_id: str,
+    project: Project = Depends(dep_project),
+    current_user: schemas.User = Depends(get_current_active_user),
+) -> schemas.ChatSession:
+    """
+    Unpin a chat session.
+    """
+
+    # Check if project is opened
+    if project.status != "opened":
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail=f"Project must be opened to unpin chat sessions. Current status: {project.status}"
+        )
+
+    # Get AgentService for this project
+    agent_manager = await get_project_agent_manager()
+    agent_service = await agent_manager.get_agent(str(project.id), project.path)
+
+    # Unpin session
+    session = await agent_service.pin_session(session_id, pinned=False)
+
+    if not session:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"Session '{session_id}' not found"
+        )
+
+    return schemas.ChatSession(**session)